1. Field Of The Invention:
This invention relates to a sealing system for use between relatively movable surfaces, such as concentric tubular members, under varying temperatures and pressures such as may be encountered in completion and production operations in oil and gas wells.
2. Description Of The Prior Art:
Establishing a seal between adjacent surfaces, such as concentric tubular members, under varying temperature, pressure and environmental conditions can be quite difficult. Seals formed of materials having substantially total elastic memory are generally desirable for use as sealing elements. However, materials which exhibit perfect elastomeric behavior under certain conditions generally do not retain these characteristics over a wide range of temperature and pressure. These materials may also be especially susceptible to attack by highly corrosive materials, such as hydrogen sulfide. Other materials, which may be too plastically deformable to be characterized as elastomeric in the strict sense of that term, may nevertheless have better sealing characteristics under hostile conditions. For example, the thermoplastic, polytetrafluoroethylene, commonly referred to as "Teflon", a trademark of DuPont, can be useful as a sealing material and also has good resistance to corrosive materials. Teflon has, however, a relatively low resiliency and high permanent set.
Polytetrafluoroethylene members have been used as sealing or packing elements in both subterranean oil well applications and to establish sealing engagement with cylindrical members in other applications. For example, Teflon packing elements have been utilized for external packer-to-casing packing element systems where the packing element is subjected to compressive preloading which energizes the element and forces it radially outward to establish sealing engagement with the casing. Teflon elements have also been employed in chevron-shaped self-energizing sealing elements. U.S. Pat. No. 3,351,350 depicts a high pressure rod seal employing a preloaded polytetrafluoroethylene ring used independently of a separate chevron ring made from neoprene impregnated duck fabric.
The prior art use of polytetrafluoroethylene in a chevron-shaped configuration illustrates the need for means to prevent axial extrusion of Teflon members and to enhance the elastic memory of Teflon elements. The use of a plurality of chevron-shaped glass-filled Teflon members in combination with a two-piece titanium metal-to-metal anti-extrusion ring was reported in Report No. SPE 6762 of the 52nd Annual Fall Technical Conference and Exhibition of the Society of Petroleum Engineers on October 9th through the 12th, 1977. Satisfactory performance of the chevron-shaped Teflon members was achieved only in conjunction with the two-piece metal-to-metal anti-extrusion ring.
The alternative problem of imparting elastic memory to the Teflon member is examined in U.S. Pat. No. 3,467,394. That patent discloses a packing assembly employing Vee-shaped polytetrafluoroethylene sealing elements interspersed with relatively rigid packing rings. The packing rings are made from flexible materials, including various flexible types of elastomers and plastics which are used to impart their memory to the Teflon sealing rings. A packing ring comprising alternating rings of thermosetting material and thermoplastic material, such as Teflon, is disclosed in U.S. Pat. No. 3,627,337. The more or less rigid non-flowing thermosetting materials were found to restrain the thermoplastic components against excessive flow.
The behavior of thermoplastic sealing elements, such as Teflon, has also led to designs in which the configuration of the tubular members in an expansion joint has been altered to improve Teflon seal performance. In U.S. Pat. No. 3,433,506, the Teflon was permitted to expand into spaced grooves on one member so that longitudinal contraction of the Teflon would promote sealing along the edges of the grooves.
The present invention also utilizes a unique geometry which, in conjunction with the thermal expansion characteristics of thermoplastic materials such as Teflon, resulting in a seal having improved performance. This seal system can be employed to establish a seal along a sealing surface of conventional tubular members such as those used in an expansion joint in a conduit or tubing string in an oil and gas well. However, this seal system can be employed in other applications.